The present invention relates to an amplification circuit for amplifying a sensing signal of a detecting element. For example, the present invention is preferably applicable to a magnetic rotational position detecting apparatus equipped with a magnetic sensing element, such as a magnetic resistive element or a Hall element, and a signal processing circuit processing the waveform of a tiny signal obtained from this magnetic sensing element.
To detect a rotational speed (i.e., a rotational angle) of an automotive engine, conventional engine speed sensors are equipped with an appropriate magnetic resistive element (MRE). The magnetic resistive element faces a gear fixed to the engine output shaft to detect the directional change of a magnetic field responsive to the rotation of the gear. The detected change of the magnetic field is converted into a tiny electric signal having a sine or sin.sup.3 waveform or a rectangular waveform. A waveform processing circuit amplifies the detected signal and compares the amplified signal with a predetermined threshold, thereby producing a binary signal (i.e., a pulse waveform signal) representing the rotational angle of the gear.
In general, the output value of the magnetic resistive element varies in accordance with temperature change. Thus, the magnetic rotational position detecting apparatus needs to have a temperature compensating function for eliminating adverse influence of the temperature change in the output value of the magnetic resistive element. For example, Unexamined Japanese Patent Application No. Kokai 2-38920 or 5-71980 discloses an operational amplifier with a gain-determining resistor having temperature dependency.
However, the temperature compensation realized by these conventional apparatuses is not satisfactory.
The magnetic resistive element has negative temperature characteristics in its output signal. Thus, the amplitude of the amplified sensor signal reduces with increasing temperature, deteriorating the S/N ratio where the S(signal) component is the output signal of the magnetic resistive element and the N(noise) component is a constant circuit error. The sensor output pulse will have worse accuracy in the angle detection.